U.S. Pat. No. 2,729,459 describes a balanced rotary power chuck having a chuck body on which a plurality of angularly equispaced jaws is radially displaceable. A central operating member is connected to the central arms of a plurality of three-arm levers each having one further arm connected to the respective jaw and another arm pivoted in a radially displaceable weight. In such an arrangement the radially outwardly effective centrifugal forces in the chuck which normally urge the jaws outwardly so as to loosen them from the workpiece or tool are compensated for by radial outward forces in the weights connected to the chuck-displacement levers which therefore serve as force-transmitting members that convert the radially outwardly effective force in the weights into a radially inwardly effective force on the jaws. This arrangement has certain advantages, yet is an extremely expensive and complex structure that has found little practical use.
Another such force-compensated chuck is described in French Pat. No. 1,008,989. Here a plurality of radially outwardly effective weights bear on an inclined surface of the operating member for the chuck jaws so that the forces that urge these jaws radially outwardly are countered by the axially directed forces on the operating member created by the weights. This arrangement, like the one described immediately above, is relatively bulky and has found little practical use.
In my earlier U.S. Pat. No. 4,026,566 a noncompensated chuck is shown wherein each of the jaws is formed with an angled recess in which engages an end part of an axially displaceable operating member. This type of jaw actuation has been found extremely effective and practical, as it not only takes up little space and can be made relatively cheaply, but it has the further considerable advantage that complete retraction of the actuators from the jaws allow them to be slid radially out of the chuck for replacement. The principal disadvantage of this structure is that excessive axial actuation of the operating member in the jaw-tightening direction can pull the actuators completely out of the recesses in the jaws, which will then immediately fly radially out of the chuck, potentially causing a serious accident.
This last-mentioned disadvantage is cured by providing a special locking-bolt structure connected to a safety ring displaceable between a jaw-removal position and a use position. In the jaw-removal position the locking bolt can be retracted out of the way of the jaw so that it can be displaced radially out of the chuck. In the use position the bolt blocks such radial outward displacement of the respective jaw so that, even if the actuators are pulled out of the recesses in the jaws, these jaws cannot move radially out of the chuck. Nonetheless this system is relatively complicated and entails considerable manufacturing expense.
In another of my earlier patents, namely U.S. Pat. No. 4,078,814, the same type of axially displaceable actuator engaged in a jaw recess is used. Here, in addition, a safety ring is provided which prevents the actuators from moving fully radially out of the chuck in the use position. This arrangement also has pivotal counterweights that can bear axially on pins that engage radially inwardly on the jaws to counteract the centrifugal outward force effective thereon. These pins, when the chuck is rotated at high speed, serve to radially lock in the jaws also as a further safety arrangement.
This last-mentioned device has proven rather difficult to use as a result of the pivotal counterweights. What is more it has been found that an occasional failure results in breaking off of the upper end of the actuator, in which case the respective chuck jaw can move radially out of the chuck, creating a very serious accident hazard. Another disadvantage of this and the other above-described known systems is that an operator will occasionally assume that the actuators are properly engaged in the backs of the jaws of the chuck when this is not the case, and will therefore start up the lathe when in fact the actuators are not properly engaged in their respective jaws. In this situation the force-conversion pin does not have time during startup to displace itself axially into radial line with the respective jaw so as to prevent it from flying out of the chuck.